(Invited) Mass Transport and Electrocatalysis: Experimental Techniques for Determining the Performance of Low Temperature Fuel Cell Electrocatalysts

Gases are important reactants in low temperature fuel cells, comprising both the fuel (hydrogen) in many systems and the oxidant (oxygen in air) in most systems. The fast transport of these reactants through the gas phase provides a problem in accurately assessing the performance of electrocatalysts over the relevant potential range of operation. This comes about because most electrochemical methods involve provision of reactant dissolved in an electrolyte, and mass transport through a condensed phase is orders of magnitude slower than through the gaseous phase. In this paper, we consider the different approaches that can be used to determine the electrocatalytic performance of test fuel cell catalysts using liquid electrolytes, and assess how closely the different approaches mimic the mass transport conditions in a low temperature PEFC. In principle, we can adopt two different approaches, we can either (a) change (in a defined way) the electron transfer kinetics of our electrochemical reaction through changing the temperature, or poisoning our catalysts; or (b) we can increase mass transport of reactant to the electrode. In the latter case, our choices include either decreasing our measurement time (fast voltammetry); increasing convection in our system (Rotating disk, Wall-Jet, channel flow, etc); or change the geometry of our system (e.g. ultra microelectrode, floating electrode etc). There is a tradeoff between accuracy of results and ease of use,  leading to some significant discrepancies in the literature.